[Bug] Content-Type text/x-component does not have charset=utf-8 encoding

Link to the code that reproduces this issue

https://codesandbox.io/p/sandbox/github/vercel/next.js/tree/canary/examples/reproduction-template

To Reproduce

1. start next.js server
2. invoke server action
3. inspect the response headers
4. content-type=text/x-component won't have charset=utf-8 encoding

Current vs. Expected behavior

I would expect that the server actions will return a correct encoding as other text based responses. This cannot be patched in proxy because the content type header is set after the proxy.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Enterprise
  Available memory (MB): 65202
  Available CPU cores: 20
Binaries:
  Node: 24.4.0
  npm: 11.4.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.1.5 // No Next.js version data was found.
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: standalone

Which area(s) are affected? (Select all that apply)

Server Actions

Which stage(s) are affected? (Select all that apply)

Other (Deployed)

Additional context

I'm willing to send a PR if you confirm that this should be fixed.